Generally, an offset PLL is mainly applied to a signal transmitting end of a wireless communication system, e.g., the offset PLL is applied to a transmitter of a Global System for Mobile Communication (GSM) system. The offset PLL directly converts a to-be-transmitted data signal to a radio frequency (RF) signal, of which power is amplified via a power amplifier for transmission. In other words, in an offset PLL transmitter, the to-be-transmitted data signal is converted from an intermediate frequency (IF) signal to an RF signal without using a conventional up-convert mixer. Accordingly, the offset PLL transmitter is capable of avoiding broadband noises caused by the conventional up-convert mixer. The to-be-transmitted data signal is carried by a plurality of sub-bands with a predetermined bandwidth, and each of the sub-bands corresponds to a predetermined frequency. Therefore, upon receiving the to-be-transmitted data signal, the offset PLL locks with respect to a sub-band of the to-be-transmitted data signal to generate a corresponding RF data signal. In other words, the controlled oscillator of the offset PLL generates an oscillating signal having a corresponding oscillating frequency according to the sub-band of the to-be-transmitted data signal. Therefore, when the to-be-transmitted data signal is switched between the sub-bands, the oscillating frequency of the oscillating signal of the controlled oscillator is correspondingly switched. Accordingly, a problem of fast and stably switching the oscillating frequency of the controlled oscillator to a predetermined oscillating frequency to transmit the to-be-transmitted data signal needs to be solved.